Key words: nanolayer, lead selenide, "negative" pressure, tangential lattice constant, forbidden gap width.

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1 ISSN: ISO 9001:008 Certified Volume 3, Issue 11, My 014 Stretching strin - effective negtive pressure in Led selenide nnolyers A.M. Pshev 1, O.I. Dvrshvili, M. I. Enukshvili, Z. G. Akhvledini,3, L.P.Bychkov, R.G. Gulyev, V.P. Zlomnov 4 1 Ntionl Avition Acdemy, Bku A-1045, Azerbijn Iv. Jvkhishvili Tbilisi Stte University, Tbilisi 018, Georgi 3 E.Andronikshvili Institute of Physics, Tbilisi 0186, Georgi 4 M. Lomonosov Moscow Stte University, Moscow , Russi Abstrct. The PbSe lyers were grown on KCl substrtes by moleculr epitxy with hot wll, the rnge of lyers thickness ws from 0 to 000nm. Specil emphsis ws given to the nnolyers < 00nm thick. The pproximtion of the tngentil lttice constnt of the lyers to tht of the substrte s the lyers thickness decreses points to the fct tht the strin (t tensile deformtion) in the lyers is ssocited with the mismtch between lyer nd substrte lttice constnts. The increses in the forbidden gp width is ssocited with stretching of lyers s well. The increse in the tngentil lttice constnt nd the forbidden gp width in IV-VI semiconductors fit well the notion of negtive pressure. Key words: nnolyer, led selenide, "negtive" pressure, tngentil lttice constnt, forbidden gp width. I. INTRODUCTION Strined lyers in semiconductor structures re successfully employed for incresing the efficiency of rdition sources nd fine tuning of their frequency over wide spectrl rnge [1]. The rnge of ppliction of strined semiconductor lyers extend significntly when these lyers re grown on heterogeneous substrtes with the mismtch of >0.01. In this cse, there pper new possibilities for designing of high-sensitivity nd high-temperture photo detectors, especilly in the infrred (IR) spectrl region, on the bsis, for instnce, of led selenide nd its solid solutions. The lyers tht re not compressed, but stretched by the substrte re of gret interest. In this cse, the forbidden gp width of IV-VI semiconductors increses. The following interesting scheme ppers: when led selenide lyers re doped with impurities such s Cr, In nd Yb, under tensile deformtion their levels shift to the forbidden gp, they compenste electriclly ctive nonstoichiometric defects, nd the concentrtion of current crriers cn decrese significntly. As result, in the nrrow-gp semiconductor, the qusi-dielectric stte is relized with preserving the chrcter of bnd-to-bnd trnsitions []. The bovementioned conditions correspond to the effective negtive pressure in the IV-VI semiconductors. The physics of negtive pressure nd crystls lys the groundwork for strongly non-equilibrium mterils science, when the lyer cn be s n efficient getter of the defects decresing entropy of lyers. This problem is likely to become the subject of much concentrted ttention in mterils nnotechnology. 318 This rticle dels with the two bsic issues of formtion of qusi-dielectric stte: the reliztion of negtive pressure nd the corresponding increse in the forbidden gp width in led selenide. The results of these investigtions lso will be employed for designing of IR lsers, piezorezsistors nd locl thermoelectric devices. This pper is orgnized in the following wy. In Sec. II we introduce the experimentl methods. The results of mesurements nd discussion re given in Sec. III. Finlly, in Sec. IV we present the conclusions. II. EXPERIMENTAL METHODS For stretching the led selenide lyers, they were grown on the substrtes with higher lttice constnt, for instnce, KCl ( =6.90, thn tht of PbSe ( =6.16, the initil mismtch mde up = 0.06 [3]. Due to the relxtion of strins, in prctice, it is difficult to chieve the deformtion t the level of initil mismtch. It is importnt to form the conditions for the mximum unrelxed stte by generting the brriers to the movement of disloctions, which just ccumulte the elstic energy of lyers. Vrious physicl nd technologicl fctors could be the brriers of this type. They include the high concentrtion of nonstoichiometric defects, nd the control of epitxy temperture [4]. The PbSe lyers on KCl (100) substrtes were grown by epitxy with hot wll. Polycrystlline led selenide ws the source of epitxy. During its synthesis, the led oxide film ws preliminrily reduced in hydrogen by heting up to 700 C, nd then the melted led ws filtered through cpillry tube. Purified components of selenium nd led were seled in qurtz mpoule nd heted up to C for 4 hours nd quenched in the ir. The temperture of the PbSe source of epitxy ws within the rnge of C. According to [5], the pressure of sturted PbSe vpor vried over the rnge of mm Hg, which llowed us to mintin the lyer growth rte in the rnge from 0.5 to 10nm/s. In this rnge, the growth rte incresed 3-fold when the distnce between the open end of the qurtz mpoule (contining the source) nd the substrte decresed from 1 mm to 1mm nd incresed times more when the source temperture incresed from 470 to 510 C. The

2 ntensity I (reltive units) Substrte temperture T, 0 С Source temperture T, 0 С Time of lyer growth t,s Tngentil lttice constnt, Å Lyer thickness d, nm Lyer growth rte ν, nm/s ISSN: ISO 9001:008 Certified temperture of the KCl substrte ws in the rnge of C. The KCl substrtes treted immeditely before epitxy (with the mximum mismtch with the plne (100) less thn 0') by cutting off were heted up t the Volume 3, Issue 11, My 014 temperture higher thn the temperture of epitxy by 100 C for n hour. Tble 1. Tngentil lttice constnts, thickness nd reltive mismtch-deformtions for epitxil PbSe lyers grown on KCl (100) substrtes (d= nm) Lyer Reltive mismtchdeformtion 1 SL (4) l PbSe 10 SL (4) SL (1) SL (0) SL (4) SL (0) SL (4) SL () SL (3) SL (3) SL (5) The negtive pressure in the lyers ws ssessed through the deformtions, which in turn were determined by mesuring the tngentil lttice constnts by X-ry diffrction ptterns of reflection from the crystllogrphic plne (400) in the mode of θ -θ scnning. The tngentil lttice constnts were determined by the formul: PbSe τ = λ/sinθ (h k l ) = λ/sinθ (1) Where λ is the X-ry rdition wvelength; θ is the Brgg diffrction ngle; h, k nd l re the Miller indices. The deformtion in the epitxil lyers with respect to the PbSe monocrystl ws determined s follows: ε = ( τ - PbSe ) / PbSe () The lyers thickness ws determined using the X-ry method by mesuring the reflection from multiple plnes (00) nd (400) with nd without the lyer. The forbidden gp width ws ssessed with opticl trnsmission recorded by using the double-bem prism diffrction spectrophotometer SPECORD 75IR in the wvelength rnge of.5-5 μm. scnning ngles Fig 1. Diffrction ptterns of scnning of thin epitxil PbSe lyers of different thickness; reflection plne (00), corresponds to the KCl substrte 319

3 Squred bsorption coefficient (cm - ) Squred product of bsorption coefficient nd photon energy (evcm -1 ) ISSN: ISO 9001:008 Certified Volume 3, Issue 11, My 014 III. RESULTS AND DISCUSSION Since the deformtions in the lyers decresed s the lyers thickness incresed, they were studied minly t thickness of less thn 00nm. At temperture <00 C, the lyers grew morphous, nd hence their growth proceeded from the temperture of 40 C, nd the time of growth decresed grdully. In Tble 1 re listed the growth conditions for some epitxil lyers, their thickness, the lttice constnts nd corresponding deformtions. It is obvious from Tble 1 tht, t the sme temperture of the source of epitxy of 470 C, while the growth time chnged from 80 to 67s, the lyers thickness chnged from 101 to 5 nm. Such chnges in the lyers thickness t insignificnt vritions in the growth time point to the dely in the initition of growth. This dely ctully turned to be the first stge of slow growth nucletion nd merging of nuclei-islnds. This process of formtion of uniform lyer took 60 s, nd then followed the second stge of fst grdul growth. The corresponding deformtions in the lyers with respect to the PbSe monocrystl mde up Lyers < 40nm thick were grown under heting up of the substrte from the rer by convlescent bulb. In this cse the growth rte of the lyers decresed significntly, nd it ws more convenient to control their thickness. By those lyers, the mximum strin - negtive pressure relized in them ws determined. For instnce, for lyer SL-177 with the tngentil lttice constnt τ = 6,19(5) Å nd the corresponding deformtion = , the strin ws E = dyne/cm =1 kbr (the modulus 1 of elsticity is E= dyne/cm, the Poisson s rtio is =0.38) nd ws developed despite of ppernce of disturbnces textures (Fig.1d). In Fig. 1(,b,c,d) re shown lso the X-ry diffrction ptterns of θ-θ scnning for the lyers of different thickness. As the lyers thickness decreses, the X-ry peks of the lyers shift to the pek of the substrte, i.e. the tngentil lttice constnts increse. It is noteworthy tht the intensity of reflection from unstrined lyer SL-69 mde up 90% of the intensity of reflection from the PbSe monocrystl. The diffrction ptterns given in Fig. 1(,b,c,d) show tht the deformtions in the lyers were determined by the mismtch between the lttice constnts of the lyer nd the substrte, nd not by the discrepncy between the therml expnsion coefficients (TEC). If the deformtions hd been ssocited with the discrepncy between the TECs, owing to significnt compression of the KCl substrte, during cooling the PbSe lyer should hve been compressed s well, nd its lttice constnt should hve been < However, up to the ngles of θ=35 corresponding to the lttice constnt 6.0, there ws detected no signl: the mismtch between the lttice constnts of the lyer nd the substrte exceeded the chnge in the lttice constnt of the lyer becuse of the discrepncy between the TECs. This mens tht the deformtions reveled in thin lyers stretched the lyer, i.e. the effective negtive pressure ws relized. Another indictor of the reliztion of negtive pressure in strined epitxil lyers of IV-VI semiconductors is the increse in the forbidden gp width with the growing deformtion in them. We hve elborted the methods of determintion of the forbidden gp width of the lyers of different thickness, including nnolyers, by opticl trnsmission [6]. At the first stge, by opticl trnsmission spectr, sequentil determintion of the refrctive index, nd the reflection nd bsorption coefficients ws performed. Then, for direct bnd-to-bnd trnsitions chrcteristic of IV-VI semiconductors, the forbidden gp width ws determined by strightening of the squred product of the interbnd bsorption coefficient nd the photon energy nd the obtined results were compred with the ones obtined by pproximtion in ccordnce with the second-type strightening α = f(hυ) α - α fr.cr. SL 55 7 (α - α fr.cr. ) h SL γ γ ,40 E g =0,453 ev 0,44 0,48 Photon energy (ev) Fig. Squired bsorption coefficient dependence on photon energy : α = f(hυ) 30 1 E g =0,451 ev 0,5 0,40 0,44 0,48 0,5 Photon energy (ev) Fig b. Squred product of bsorption coefficient nd photon energy dependence on photon energy : (αhυ) =f(hυ)

4 Forbidden gp width Eg (ev) Lyer thickness d, nm Tngentil lttice constnt τ, Å ISSN: ISO 9001:008 Certified The spectr of epitxil lyers <700nm thick generlly showed monotonic increse with smll slope s the wvelength incresed. In this cse, s well s in the cse of thick lyers in the spectr of which interference peks were reveled, such spectr were processed by model of the Fbry-Perot interferometer. Tking the epitxil PbSe lyer s resontor the ends of which re the boundries with the substrte nd the ir, the model considers the interference s the one dmping in the wve lyer under multiple reflection from the resontor boundries. The mgnitude of the trnsmission coefficient through the lyer is the following: T = ( 1 - r 1 )(1 r ) exp ( β i d ) / [ ( 1 R ) + 4R sin β d ] Where r 1 = N PbSe N ir. / N PbSe + N ir. is the mplitude vlue of reflection coefficient t the PbSe/ir interfce; r = N PbSe N KCl / N PbSe + N KCl is the mplitude vlue of reflection coefficient t the PbSe (lyer) - KCl (substrte) interfce, R = r 1 r exp ( β i d), β = π N / λ, β i = - α /. is the bsorption coefficient. Extrpolting the vlues of the refrctive index determined by interference peks, for instnce, for lyer (3) Volume 3, Issue 11, My 014 SL μm thick, the vlues nd in the shortwvelength region were obtined [6]. The obtined vlues of nd were substituted into the eqution for determintion of the bsorption coefficient α through y= exp(-αd): 1 1 T r r y 1 r 1 r y Nd Tr1r 4Tr1r sin y T 0 Becuse of the high concentrtion of current crriers (~10 19 cm -3 ) in the epitxil lyers, the bsorption by free crriers is high, nd it could be quite significnt in the region of eigen bsorption s well. Hence the strightening of squred bsorption coefficients is performed by the reltionship fhν. Tble. Dt for the tngentil lttice constnts, deformtions (strins) nd corresponding forbidden gp width in PbSe lyers. Lyer Reltive mismtchdeformtion PbSe PbSe fr.cr Strin E 1, 1 SL (4) SL (0) , SL (8) , SL (4) , The quntity α-α fr.cr corresponds to interbnd bsorption nd chrcterizes its edge, especilly in thin lyers where, kbr (4) E g, ev long with the high concentrtion of current crriers, significnt decrese in the relxtion time is observed Tngentil lttice constnt а (Å) Fig 3 Dependence of the forbidden gp width of PbSe lyers on the tngentil lttice constnt 31

5 ISSN: ISO 9001:008 Certified As t high concentrtions of current crriers the trnsitions between bnd edges re impossible becuse of the Burstein effect, for determintion of the forbidden gp width, the strightening of function 1 fh fr.cr. Volume 3, Issue 11, My 014 is performed. In this expression, ccounts for degenertion, nd the dependence of this fctor on the forbidden gp width nd the Fermi level is given in Ref. [7]. Figure,, b shows the reltionships (α hν) = f(hν) nd ( ) f ( h ) for lyer SL nm thick with the tngentil lttice constnt τ =6.177(4) Ǻ. Attention ws pid to the fct tht, in the prticulr spectrum region of μm, the interbnd bsorption exp. fr. cr. ws positive, nd, for direct bnd-to-bnd trnsitions, it vried in the intervl from to 10 4 cm -1. Proceeding from this point, the reltionships (α) = f(hν) nd were constructed [8]. In Tble re presented dt on the forbidden bnd gp t T=300K with corresponding tngentil lttice constnts nd deformtions. The vlue 4.4 cm /Vs ws tken s the verge mobility of current crriers, while, in the clcultion of coefficient γ, the vlues E g0 =0.86 ev nd E F =0.03 ev were used. As is evident from Fig.,, b, the strightening of both types yielded for E g ~0.453 nd ev, respectively. These vlues differed from the E g = ev determined by the minimum of the reltionships given in Fig.,, b within ev, which ws less thn the experimentl error (~0,005eV). Proximity of the E g vlues by both types of strightening could be explined by the fct tht the photon energy chnged only slightly t the strightening section. The E g vlue incresed by ~ 0.17 ev s compred with the unstrined lyer. The estimtion showed tht, t the given lyer thickness nd the sub grin size ~ 50 nm [9,10], the increment of the forbidden gp width ws determined by deformtion nd not by quntum effects (if effective mss in the nnolyers is not chnged significntly). Figure 3 shows the dependence of the forbidden gp width on the tngentil lttice constnt. The deformtion REFERENCES [1] A.M. Pshev, O.I. Dvrshvili, M. I. Enukshvili, R.G.Gulyev, V. P. Zlomnov. Supercriticl Nnostructures PbSe/KCl, GEN, 011,, [] A.M. Pshev, O.I. Dvrshvili, V.A. Aliyev, M. I. Enukshvili, V. P. Zlomnov. Problems of Mismtch in the Heterostructures Bsed on IV-VI Semiconductors. In: Progress in Science nd Engineering of Modern Avition, 009, 1, [3] A.M.Pshev, O.I.Dvrshvili, Z.G.Akhvedini, M.I.Enukshvili, R.G.Gulyev, V.P.Zlomnov. Unrelxed 3 corresponding to the lttice constnts of, for instnce, 6.149(0) Ǻ - lyer SL-77 nd 6.177(4) Ǻ - lyer SL-55 mde up nd 0.008, respectively. It is obvious from Fig. 3 tht, with growing deformtion, the forbidden gp width incresed more rpidly, lmost twice s much, s compred with its chnges in the row SL-69 SL-77 SL- 55(Tble ). IV. CONCLUSIONS The monoclystlline nd uniform PbSe lyers were grown on KCl substrtes by moleculr epitxy with hot wll. The dependences of the deformtion on lyer thickness nd of the forbidden gp width on the tngentil lttice constnt were studied. Specil emphsis ws given to the nnolyers <00nm thick. The deformtions in the lyers were determined by mesuring the tngentil lttice constnt. It ws estblished tht with the decrese of the lyers thickness the deformtion (stretching) in the lyers incresed nd, for instnce, for the lyer nm thick the deformtion ɛ ws nd corresponding strin - 1 kbr. It ws shown tht deformtions in the lyers were connected with the mismtch between the PbSe lyer nd the KCl substrte (more ccurtely tht is fter strin relxtion t the lyersubstrte interfce). The effect of stretching of the PbSe lyer effective negtive pressure ws lso reveled when the forbidden bnd gp ws determined by studying of the opticl trnsmission. The opticl spectr were processed by model of the Fbry-Perot interferometer. The methods of sequentil determintion of the index of refrction, reflection nd bsorption coefficients nd with tking into ccount the degenercy of current crriers were developed. The vlues of the forbidden bnd gp determined by the strightening of two reltionships (α) = f(hν) nd (αhν) = f(hν) coincided nd were equl to 0.454eV t the deformtion ɛ = In this cse the increment of the forbidden bnd gp ws 0.17eV in comprison with the unstrined PbSe lyer nd on the whole this increment is determined by deformtion nd not by quntum effects. These investigtions nd the obtined results re n essentil bse for the cretion of new IR high-sensitivy nd hightemperture photo detectors, tunble lsers by opticl pumping nd locl thermoelectric devices by doping PbSe (or other compositions of IV-VI semiconductors) nnolyers. Stte in Epitxil Heterostructures Bsed on Led Selenide.J.Mod.Phys. 01, 3(6), [4] Z. G. Akhvledini, O.I. Dvrshvili, A.M. Pshev, M. I. Enukshvili, R. G. Gulyev. On the Contribution of Point Defects to the Formtion of Supercriticl Nnostructures Bsed on Led Selenide. In: Book of Abstrcts of the 16 th Interntionl Conference on Rdition Defects in Insultors, Beijing, 011, 16. [5] V.P.Zlomnov, B.A.Popovkin, A.V. Novoselov. Determintion of the Pressure of Sturted Vpor of Solid Led Selenide. J. Inorg. Chem., 1959, 4, 1,

6 ISSN: ISO 9001:008 Certified [6] A.M. Pshev, O.I. Dvrshvili, Z. G. Akhvledini, M. I. Enukshvili, L. P. Bychkov, M.A. Dzgni.Study on the Forbidden Gp Width of Strined Epitxil Led Selenide Lyers by Opticl Trnsmission. J. Mt. Sci. Eng., 01, B(), [7] E. Plik, D. Mitchel, J. Zemel. Mgneto-Opticl Studies of the Bnd Structure of PbS. Phys. Rev., 1964, 135A, A763-A768. [8] A.M. Pshev, O.I. Dvrshvili, Z.G. Akhvledini, M. I.Enukshvili, L.P.Bychkov, M.A. Dzgni.Interprettion of the Opticl Trnsmission Spectr of Led Selenide Lyers with the Aim of Determintion of the Width of Their Forbidden Gp.Trns. Azer.Nt.Avition Acd. 013, 15, 3. [9] A.M. Pshev, O.I. Dvrshvili, M. I. Enukshvili, R.G.Gulyev, M.A.Dzgni, V.P.Zlomnov. Investigtion of the Degree of Dispersion of Epitxil Led Selenide Lyers. Bull.Georg.Nt.Acd.Sci. 011, 37(1), 3-6. [10] A.M. Pshev, O.I. Dvrshvili, Z.G.Akhvledini, M.I.Enukshvili, R.G.Gulyev, V.P.Zlomnov.The Structure of Thin Epitxil Lyers of Led Selenide.J.Mt.Sci.Eng., 013, 3(), Volume 3, Issue 11, My

Lesson 8. Thermomechanical Measurements for Energy Systems (MENR) Measurements for Mechanical Systems and Production (MMER)

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